Most
people would be happy to get rid of excess body fat. Even better: Trade
the spare tire for something useful — say, better-functioning knees or
hips, or a fix for an ailing heart or a broken bone.The idea is
not far-fetched, some scientists say. Researchers worldwide are
repurposing discarded fat to repair body parts damaged by injury,
disease or age. Recent studies in lab animals and humans show that the
much-maligned material can be a source of cells useful for treating a
wide range of ills.At the University of Pittsburgh, bioengineer Rocky Tuan
and colleagues extract buckets full of yellow fat from volunteers’
bellies and thighs and turn the liposuctioned material into tissue that
resembles shock-absorbing cartilage. If the cartilage works as well in
people as it has in animals, Tuan’s approach might someday offer a kind
of self-repair for osteoarthritis, the painful degeneration of cartilage
in the joints. He’s also using fat cells to grow replacement parts for
the tendons and ligaments that support the joints.Foremost among
fat’s virtues is its richness of stem cells, which have the ability to
divide and grow into a wide variety of tissue types. Fat stem cells —
also known as adipose-derived stem cells — can be coerced to grow into
bone, cartilage, muscle tissue or, of course, more fat.

Multipurposes

Cells from fat are being tested to mend tissues found in damaged joints, hearts and muscle, and to regrow bone and heal wounds.

The
stem cells in fat share the medical-worthy spotlight with a few other
cells. Along with the fat-filled adipocytes that store energy, fat
tissue has its own blood supply and supporting connective tissue, called
stroma. The stroma contains blood cells, immune cells, endothelial
cells that line the inner surface of blood vessels and pericytes, which
line the outer surface. These other fat-derived cells are proving to
have therapeutic value as well.

Plastic surgeon J. Peter Rubin,
also at Pitt, says that the multitalented cells found in fat could
prove to be the ultimate body repair kit, providing replacement tissue
or inspiring repair of body parts that can’t mend themselves.

Much
of the research — more than a decade of studies — has been in lab
animals, but a few applications are being tested in human volunteers.
Current clinical studies under way aim to provide replacement tissue to
treat chronic wounds and diabetic sores, or conditions such as
Parkinson’s disease, multiple sclerosis, chronic obstructive pulmonary
disease and type 1 diabetes.

Most clinical studies use the
simplest approach: Harvest cells from a patient, then inject them in a
single procedure. In more complex approaches still in lab and animal
testing, various cells in fat are extracted and manipulated to create
custom treatments for worn-out or damaged tissues or to generate blood
flow after a heart attack or replace bone in large fractures.

Questions
remain, however, about how the cells do their regenerative magic.
Scientists and regulators still have plenty to figure out, such as what
cell characteristics work best for each application.

A lush source

Stem
cells can develop into various cell types, which makes them the focus
of studies that aim to replace cells that fail because of disease,
accident or age. Stem cells taken from embryos are more versatile than
other types of stem cells, but their use is controversial. For that
reason, researchers have studied stem cells from sources other than
embryos, including bone marrow, muscle and blood.

Fat tissue comes
from the same embryonic tissue as bone marrow, a traditional stem cell
source, so scientists reasoned that fat might contain similar cells. In
2002, UCLA researchers discovered stem cells in human fat. They were surprised to find vast quantities.

Stem
cells make up 2 to 10 percent of fat tissue. A cubic centimeter of
liposuctioned fat (about one-fifth of a teaspoon) yields 100 times as
many stem cells as does the same amount of bone marrow, Tuan says. And
fat cells are easy to harvest — much easier than bone marrow. One pound
of fat removed from a patient’s abdomen can yield up to 200 million stem
cells, a more than adequate supply for treatments.

Why fat
produces so many stem cells isn’t clear, but Rubin points out that fat
tissue serves several important functions. In addition to storing and
releasing energy, it helps insulate and protect the body’s internal
organs. “Like most tissues in the body, fat has a reservoir of stem
cells to replenish cells as they die off or create new cells in response
to growth or the need for more cells,” he says.

Fat produces so
many stem cells, in fact, that for some applications — such as
tissue-replacement or “fat grafting” — there’s no need to grow more of
them in the lab. Once harvested, liposuctioned material is treated with
enzymes to remove cells from the surrounding tissue, then put into a
centrifuge to separate the stem cells from other cell types. In about an
hour, the stem cells are ready to be injected back into the patient to
plump skin or round out fat tissue lost to injury or disease. Rubin has
used this method to treat patients who have lost tissue during breast
cancer surgery or have been injured in war. His lab is conducting a
clinical trial on the use of fat stem cells to plump up tissue at the
site of an amputation to improve the comfort and fit of a prosthetic arm
or leg or to make it easier to tolerate sitting for long periods in a
wheelchair.

Already, Rubin’s team has treated five military
patients, extracting fat from each patient’s abdomen and injecting the
stem cells back into the patient at the injury site. He and other
scientists think that the fat stem cells remodel tissue by releasing
growth factors and communicating with surrounding cells in their new
location — sending and receiving signals through chemical cues. As a
result, the stem cells enhance the growth of new fat tissue and boost
blood supply to surrounding tissue. Over a period of several weeks, the
cells he injects form a mound of fat tissue, allowing patients to fit a
prosthesis or sit without pain. So far, all of the patients have
benefited from the stem cell injections, he says, though his group is
still working on how much to inject for each patient.

What's in fat?

Fat
is an organ with a complex assembly of cells. In addition to fat cells,
or adipocytes, and blood vessels, fat tissue contains stem cells,
pericytes (cells that stabilize blood vessel walls), pre-adipocytes
(precursors to fat cells), macrophages (immune cells) and endothelial
cells, which form the inner lining of blood vessels.

M. Telfer

Stretching limits

Other
applications require manipulating cells in the lab, placing fat stem
cells in a specific environment — and sometimes putting mechanical
pressure on them — to direct the cells to transform into certain cell
types.

Tuan’s group at Pitt places fat stem cells on scaffolds
that help guide the growth of the cells, developing treatments to
regenerate anterior cruciate ligament tissue or to repair rotator cuff
injuries and Achilles tendon ruptures. Injury to ligaments and tendons
is common, especially among athletes, but tears or worn-down areas
generally don’t heal completely by themselves. Efforts to create
substitute tissues have largely failed, Tuan says, because re-creating
the structure of a tendon or ligament remains a challenge.

Tendons
are the cables that connect muscle to bone, allowing arms to rotate at
the shoulder, knees to bend or fists to clench. Cells in tendons, called
tenocytes, line up along long fibers of collagen, creating molecular
bridges that reach across and intertwine with collagen cables to help
give them strength and flexibility. This structure allows tendons to be
stretched up to 15 or 20 percent beyond their original length and snap
back into shape.

Tuan’s group has discovered a trick for turning fat stem cells into tenocytes that grow in the same organized way. In 2013, the researchers outlined the method in Biomaterials.
To replicate the structure of natural tissues, the scientists created
scaffolds of biodegradable nano-sized fibers. Fat cells were then
combined with bovine collagen and placed, or seeded, into the scaffold.
The tiny fibers interacted with the stem cells, sending and receiving
instructions that guided the stem cells’ growth. Over seven days, as the
stem cells differentiated into tenocytes, the scientists applied
mechanical force on the ends of the scaffold — pulling the structure to
keep the cells under tension just like a natural tendon would do during
motion.

By
tugging on fat stem cells, Tuan says, the group can create replacement
tendons that are strong, stiff and resilient, like natural human
tendons.

Tuan’s group is also exploring 3-D printing to create artificial cartilage from fat stem cells.

Printing parts

Cartilage
is a flexible tissue that serves as padding between bones, allowing
knees, fingers, hips and shoulders to move freely. When cartilage wears
down, the result is osteoarthritis, a painful condition that affects one
in four people, often those over age 65.

Once cartilage is
damaged, it continues to deteriorate, forming what Tuan calls
“potholes.” Over time, the potholes grow, eventually reaching the bone.
The standard solution is a joint replacement. In the United States, more
than 1 million people get knee or hip replacements each year.

Tuan
calls the process “rebuilding the road.” The invasive procedure
requires surgically replacing the joint with plastic and metal parts
that generally last 10 to 15 years. Because an increasing number of
people get new joints in their 40s or 50s, many require more than one
round of surgery. “But there’s a limit to the number of times you can do
that,” he says.

Tuan’s 3-D printing method builds thin layers of
fat stem cells into a custom-sized scaffold to create new cartilage in
the size and shape needed. The “ink” is made of fat stem cells plus
gelatin, which consists of proteins found in living tissue. The
scientists chemically modify the gelatin so that the ink remains fluid
during printing. Once printed, the material is irradiated with light so
that enzymes in the mixture form bonds, cross-linking to create stiffer,
cartilage-like material.

The procedure has been used to create cartilage implants for rabbits and goats. Animals that once hobbled were able to hop, trot and otherwise move about, according to a report last August in Frontiers in Bioengineering and Biotechnology.

“Because
the engineered cartilage is a living tissue … unlike a metal or polymer
implant, it is expected to continue to grow into its natural shape and
function once it is implanted into the joint,” Tuan says. “No
replacement is therefore necessary.”

Still, it’s not the ideal
solution, Tuan admits. “The problem is that’s not how tissues are
formed,” he says. “Tissues form when cells migrate to a place, make
themselves at home and build their own support structure, or matrix.”

His
group is now devising ways to allow fat stem cells to set up their home
right at the site of the pothole. The vision is to create a minimally
invasive procedure, giving doctors a tool they can thread through a
catheter to print the fat-derived stem cell cartilage at the site of the
damage, inside the joint. Fat stem cells could then settle in and
multiply directly in the joint. Additional arthroscopic instruments,
also under development in Tuan’s lab, will allow physicians to guide the
injection and smooth out newly printed cartilage to create a perfect
fit that closely resembles the real thing.

So far, each step in the new approach has been developed. The next step is to tie all the pieces together in animal studies.

Boning up on body repair

The
body does a better job of healing broken bone than healing cartilage.
But if the fracture is large or a significant amount of bone is lost,
the bone may not heal. In such cases, surgeons can take bone from
another part of the patient’s body, or use bone from a cadaver, to fill
in the gap. Biomedical engineer Warren Grayson
of Johns Hopkins University says more than 1 million bone replacement
procedures are performed each year in the United States, often after
accidents or tumor removal. The surgery is invasive and carries risks of
rejection, infection and lingering pain.

A better option, Grayson
says, is to help patients grow bone from their own fat cells. Because
the bone-growing material comes from the patient’s body, the grafts are
less likely to get rejected than cells from donor tissue. What’s more,
the bone may later grow with the patient, potentially eliminating the
need for multiple surgeries in children who receive grafts but still
have growing to do.

Since 2010, Grayson’s team has been growing
bone from liposuctioned fat and successfully implanting the bone in
animals. Stem cells taken from fat are placed in a bioreactor, an
incubator--like device that nourishes cells as they grow on a scaffold
for five weeks. Added nutrients and growth factors help the cells
transform into bone cells.

Already, fat stem cells have been used
in a few trials to help regenerate bone in people. In 2004, German
doctors successfully used stem cells collected from a 7-year-old’s fat,
along with other cells, to repair damage to her skull. Five years later, scientists at Cincinnati Children’s Hospital Medical Center seeded a bone graft with fat stem cells to replace a teen’s missing facial bones.

After
14 days in a scaffold with growth factors, human endothelial cells from
liposuctioned fat (top) developed into a network of blood vessels and
wrapped around scaffold fibers. A sample 3-D printed scaffold mimics a
lower jawbone (bottom).

Both: J. Temple et al/Journal of Biomedical Materials Research 2014

In
the case of the teen, fat stem cells were injected onto a scaffold from
donor bone. But such bone grafts require multiple surgeries and don’t
come with a ready blood supply to nourish the new bone as it grows.

Grayson’s
group aims to make the repair process easier on the patient. He and his
team are growing fully functioning bone — with its own blood supply —
from fat. Each graft can be custom-designed, using 3-D modeling and
printing, to fit precisely where needed.

His team is experimenting
with different formulas — and two different cell types from fat — to
find the best ways to form all the cell types needed.

More recently, his group tested fat-derived stem cells against bone marrow cells in creating new bone. The fat stem cells outperformed the bone marrow stem cells. The findings, published in the September 2015 Stem Cells,
show that in the presence of specific growth factors over a period of
weeks, fat stem cells produced more calcium and bone mineral deposits
per cell than did the bone marrow stem cells.

The current
challenge is to produce tissue that has its own system of blood vessels
to supply nutrients needed for the new bone to grow. In the Journal of Biomedical Materials Research Part A in 2014, Grayson’s group outlined a method for printing bone grafts
with internal pore structures that would allow blood vessels to grow
through the graft while maintaining the structure of the scaffold. The
team is now investigating ways to help spur such growth by seeding the
structure with endothelial cells or blood-vessel forming cells from fat.

Heart-healthy fat cells

Endothelial
cells and other cells in fat are the lifeblood of efforts to develop a
patch that can be applied to damaged heart tissue following a heart
attack. Stuart Williams,
a cardiologist at the University of Louisville in Kentucky, is creating
a cell-infused patch seeded with a mixture of smooth muscle cells,
endothelial cells and blood cells, all obtained from fat tissue.

The idea for the patch, outlined in 2013 in Stem Cells Translational Medicine,
is to harvest fat from a patient, pull out the vessel-forming cells and
seed the cells onto a biomaterial that can be immediately implanted.
The whole process, from start to finish, will take about an hour.

The
fat-cell patch works particularly well to promote healing in very small
blood vessels, Williams says, a feature that may be especially
beneficial for women, who often have more problems with their small
blood vessels and fewer problems with the large ones.

Repairing heart damage

Rats
treated with a patch seeded with endothelial cells from fat tissue two
weeks after a heart attack (MI SVF) show more new blood vessels (green)
in the damaged area of the heart, compared with untreated rats (MI) and
rats treated with an unseeded patch (MI Vicryl). Four weeks after
treatment, the MI SVF rats had better heart function and less tissue
damage. Total vessel density (vessels per square millimeter) was
determined through staining methods.

Source: A.J. Leblanc et al/Stem Cells Translational Medicine 2013

“The
interesting thing is, there’s really no stem-ness to these cells at
this point,” Williams says. “They don’t have to differentiate. All they
have to do is reconnect with each other to form these new blood
vessels.”

The patches could be created in the operating room during surgical bypass, he says.

Such
patches might also be applied to other areas of the body, such as legs,
hands or feet, where patients have limited blood flow, Williams says.
In wounds that aren’t healing well, cells could be injected directly
into the area to promote blood flow and healing.

While Williams
has shown his technology works in animals, he hasn’t yet tested it on
people. Getting the federal go-ahead to pursue studies in humans remains
a challenge for the heart patch and many other new applications of fat
cells.

Current guidelines issued by the U.S. Food and Drug Administration
allow trials for treatments in which cells are harvested and injected
back into the same patient in a single surgery. But the FDA, and
regulatory agencies worldwide, are wrangling with questions on how to
test and assess new types of therapies in which cells are grown on
scaffolds or manipulated in the lab.

Questions remain, for
example, on how to best handle cells in the lab to ensure safety and
purity of a product, and how to package and transport products once they
are made. Fat stem cells, for example, may change or dedifferentiate
when growing in a lab dish, sitting on a warehouse shelf or even
following injection into the body, Tuan says.Later this year, the
FDA plans to hold a public hearing to solicit comments from scientists,
manufacturers and others on how to proceed. Meanwhile, scientists in
the field agree that the potential for fat to do good is here.

“Fat
may actually be a natural storehouse of regenerative cells,” Williams
says. “When applied correctly, these cells may someday help repair
bodies on an as-needed basis.”This story appears in the March 19, 2016, issue with the headline, "Fat as a fixer."